To operate the bubble memory, current pulses of predetermined conditions must be fed to various electrically conductive patterns, such as a generator, an annihilator, a replicator, a transfer switch gate, and the like. Further, the operating regions of the biasing magnetic field for these current pulse conditions serve as important factors for expressing the operation characteristics of the bubble memory. For instance, the change in characteristics of a bubble memory relative to pulse current conditions applied to a generator is considered below. The pulse current will have phase characteristics, pulse width characteristics and current characteristics. If these characteristics exist independently of each other, the bubble element will properly operate within given ranges of phase, pulse width and current value of the pulse current.
In practice, however, the bubble element does not properly operate even when various signal components, such as phase, pulse width and current value, satisfy the above-mentioned ranges. Through experiments it has been learned that the characteristics do not exist independently of each other. For example, a pulse current is directly affected by the phase, biasing magnetic field and pulse width. Consequently, the variation ranges cannot be guaranteed if the characteristics are tested independently of each other. Although the above description has dealt with the characteristics of a pulse current applied to a generator, the same fact also holds true for other functional gates.
In testing the bubble memory, therefore, the operation cannot be completely guaranteed unless the allowable variation regions are guaranteed while taking the mutual effects into consideration.
For this purpose, as will be mentioned later in detail, an element must be tested in regard to whether or not it properly operates for a maximum value, a minimum value and a standard value of phase, pulse width and current value of a pulse current fed to all of the functional gates.
Combinations of these phases, pulse widths, current values and functional gates must be set in a digital manner while being controlled by an electronic computer, and must be tested successively and repeatedly.
Thus, if it is attempted to test the testing elements consisting of various functional gates, a tremendous number of combinations must be treated, requiring great periods of testing time. If the individual testing elements are measured with respect to only two or three points, then the operating conditions in the regions between such points cannot be guaranteed. Therefore, among the elements which have passed the above-mentioned time consuming test a significant number of elements turn out to be defective after they have been put into practical use. This is attributed to the fact that when particular values of the test elements are combined, the elements may properly operate under the condition of such a combination, but fail to properly operate under the conditions of other combinations. It can be empirically foreseen to some extent which combination of which values of the test elements is most likely to develop erroneous operation or, in other words, which combination provides the narrowest or the poorest operating margins. In practice, however, the design requirements and variance depending upon the lots, make it difficult to foresee desirable combinations. If the worst combinations are known, they could be taken into consideration in planning the test. With the worst combinations unforeseeable, however, there is no way to cope with the situation.